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Mg3Si4O6 Theoretical (63.7% SiO2, 28.4% MgO, 6.98% Cao) High temperature magnesia matte glazes employ MgO from talc and magnesium carbonate to form magnesium silicate crystals on cooling to give both opacity and a matte silky surface. Some textbooks claim that talc is used as a low fire body addition to encourage conversion of excess free quartz to cristobalite to increase body expansion which reduces crazing. While the ease of glaze fit associated with talc bodies appears to confirm this, Ron Roy points out that his testing indicates that cristobalite does not form at cone 04 or below and that his dilatometer curves show no cristobalite in such bodies. Talc is also used to produce thermal shock resistant stoneware bodies where it acts as a low expansion flux that reduces body expansion by converting available quartz mineral, mainly in kaolin, to silicates of magnesia. Cordierite bodies used in kiln furniture and flameware extend this concept so that all free quartz is used up. Such bodies tend to have a narrow firing range because all the flint needs react before the body distorts. Talc is a curious material in that, by itself it is a refractory powder; yet in amounts of 1-5% in middle temperature stoneware bodies it can drastically improve the maturity and melting. In ceramic slips, where 50% is often used, it produces a body that melts suddenly by cone 4. In glazes at middle temperature talc does not participate much in the melt and its presence tends to create an opaque silky matte surface, at cone 10 it is a powerful flux. Source: Digital Fire.